Ion source utilizing a spherically converging electric field



D. GABOR Nov. 15, 1966 ION SOURCE UTILIZING A SPHERIGALLY CONVERGING ELECTRIC FIELD 5 Sheets-Sheet 1 Filed Oct. 16, 1961 AXIS OF CYLINDRICAL FURNACE OR ION SOURCE INVENTOR. DENNIS GABOR BY fl 4 3% WM /M ATTORNEYS D. GABOR Nov. 15, 1966 ION SOURCE UTILIZING A SPHERICALLY CONVERGING ELECTRIC FIELD 5 Sheets-Sheet 2 Filed Oct. 16, 1961 FIG. 2

INVENTOR.

DENNIS GABOR Nov. 15, 1966 D. GABOR 3,286,187

ION SQURCE UTILIZING A SPHERICALLY CONVERGING ELECTRIC FIELD Filed Oct. 16, 1961 5 Sheets-Sheet 5 FIG. 3

FIG. 8A

INVENTOR.

DENNIS GABOR BY W/4SW Mu/M ATTORNEYS Nov. 15, 1966 D. GABOR 3,286,187

ION SOURCE UTILIZING A SPHERICALLY CONVERGING ELECTRIC FIELD Filed Oct. 16, 1961 5 Sheets-Sheet 4 FIG.8

IN VEN TOR. DENNIS GABOR BYWASW ATTORNEYS ION SOURCE UTILIZING A SPHERICALLY CONVERGING ELECTRIC FIELD Filed Oct. 16, 1961 D. GABOR Nov. 15, 1966 5 Sheets-Sheet 5 INVENTOR.

DENNIS GABOR BY W A. SW JM ATTORNEYS United States Patent 3,286,187 ION SOURCE UTILIZING A SPHERICALLY CONVERGING ELECTRIC FIELD Dennis Gabor, London, England, assignor to Minnesota Mining and Manufacturing Company, St. Paul, Minn., a corporation of Delaware Filed Oct. 16, 1961, Ser. No. 145,317 Claims. (Cl. 328-233) This invention relates to a novel and very useful apparatus and a method for producing ion beams.

More specifically, this invention is directed to an ion source and electron-optical system for the production of ion beams and to methods for using the same.

My combined ion source and electron optical system is capable of operating with a wide variety of ions and is especially useful for the production of a scanned highdefinition picture of the order of about 0.5 square centimeter. It is also capable of reducing an etched mask about 20 times or perhaps even more. A further capability of the system is in machining by ions, for it generally produces deeper and better defined holes and slots in metals and the like than electron-machining. Other uses and advantages of my system will be apparent to those skilled in the art.

The invention is better understood from the following description of which these drawings are a part:

FIGURE 1, parts A and B, shows a spherically converging electric field used in this invention to produce an ion current of uniform energy.

FIGURE 2 shows the electric field and ion trajectories associated with apparatus of this invention.

FIGURE 3 is a cross-sectional assembly drawing of a first embodiment of the ion source and first lens used in apparatus in this invention.

FIGURE 4, parts A, B and C, shows details of a cathode assembly suitable for use in apparatus of the invention.

FIGURE 5 shows details of an accelerating grid suitable for use in apparatus of the invention.

FIGURE 6 shows details of an inner heater helix suitable for use in apparatus of the invention.

FIGURE 7, parts A and B, shows details of an outer heater assembly suitable for use in apparatus in the invention.

FIGURE 8 is a partial top view of a second embodiment of the ion source and first lens used in apparatus of this invention.

FIGURE 8A is a cross-sectional view of a second embodiment of the ion source and first lens used in apparatus of this invention, taken along a diameter cut on the plane of the lead wires for the cathode and grid.

FIGURE 9 is a diagrammatic representation using an optical analog to show how a scanning operation is conducted using an ion beam produced in accordance with the invention.

FIGURE 10 is a diagrammatic representation using an optical analog to show how a transparency is reduced as for recording using an ion beam produced in accordance with this invention.

FIGURE 11 is a longitudinal, cross sectional view of an assembly containing a field lens and a projector lens suitable for use in apparatus of this invention.

To generate an ion current which is as large as possible and at the same time uniform in energy, I employ electron impact to ionize vaporized atoms. In addition, and most importantly, I develop a spherically converging electric field in the volume wherein such ion current is generated.

Such a spherically converging electric field I preferably create by means of an unevenly wound heating helix Patented Nov. 15, 1966 circumferentially arranged about a cylindrical space or cavity wherein the ion current is produced.

However, those skilled in the art will appreciate that other equivalent means can be used to produce a spherically converging field (converging into the center of the nozzle). Thus, for example, one can use a succession of rings at diiferent potentials. The unevenly wound helix is of course the simplest means.

Briefly, the invention operates as follows: Electrons are produced at a hot dish-shaped electron emitting cathode such as a Coolidge spiral and accelerated through a grid. Beyond the grid the electrons bombard ionizable atoms thereby generating ions. These ions are accelerated away from the region of the cathode and grid towards the opposite end of the chamber by means of an electric field. By making this field increasingly stronger as the distance from the grid increases, I cause these ions to converge spherically to a point along the axis of the cylindrical furnace cavity. I cause this point of convergence to fall below the cylindrical furnace cavity but above the entrance to an electron optical system employing three electrostatic lenses (i.e., in the region of a nozzle on the axis of the cathode and of the grid). Any suitable known means can be used to produce this approximately spherically converging electrostatic field between the grid and the nozzle. The nozzle itself is maintained at the same potential as the cathode. The ions entering this electron optical system are collimated, accelerated and focused into a true ion beam.

The efiiciency of ion generation in the furnace cavity is further enhanced by using low energy electrons which are caused to pendulate axially in the cylindrical furnace cavity. The result is that a much greater fraction of electrons collide with neutral atoms and molecules to produce ions than would otherwise be the case if high energy non-pendulating electrons were employed to ionize. I cause the electrons to pendulate axially in the region between accelerating grid and nozzle by means of a substantially axial magnetic field produced by coils circumferentially positioned about the cylindrical furnace cavity. While so-called penning type ion sources in which the electrons pendulate between two cathode potential electrodes are known, the electrons in such prior art device pendulate at right angles to ion stream. At the low gas or vapor pressures preferentially used in my device, the mean free path for ionizing collision is rather long. By having the electrons pendulate between the cathode and the nozzle of the furnace area, the electrons perform many pendulations before being absorbed by the grid, thereby vastly increasing the probability that any given electron will produce an ion through collision.

A further improvement in my ion source and gun system is realized by the incorporation of a high temperature vacuum furnace so that it becomes possible touse as ionizable materials substances which have exceedingly low vapor pressures, such as boron or carbon. Thus, my ion source can efiiciently operate for the production of ion currents using very low vapor densities within the furnace. Such an arrangement enables one to handle all sorts of materials as sources for the generation of ions.

I find that it is unexpectedly advantageous for the production of dense, uniform ion beams to use, in combination, a spherically converging electric field in combination with an electromagnetic field, as described herein.

The ion source of my invention will now be described. To generate an iron current which converges to a small area base 0 (as shown in FIG. 1A) and at the same time produce a current which is uniform in energy, I employ a spherically converging electric field surrounding the cylindrical cavity 52 (see FIG. 2) wherein ions are created. Such cavity 52 is enclosed by a refractory ceramic tube 64 into which is fitted an unevenly wound helix 58, composed preferably of tungsten wire 59 (see FIG. 6). This helix 58 is wound so that when energized, a spherically converging electric field is set up which causes ion convergence at point in FIG. 1A.

The action of the spherically symmetrical electric field set up by inner heater helix 58 is understood by reference toFIG. 1. The electric field set up by helix 58 produces the conically converging ion trajectories shown by lines 23. Within the assembled ion source furnace 18 (FIG. 2) regions of equipotential are suggested by lines 22 (FIG; 1A). A plot of the potential distribution along the axis of ion source furnace 18 is shown in FIG. 1B.

The action of the furnace is as follows. Electrons are produced by cathode 20 and accelerated by accelerating grid 21. In the ion source cylindrical furnace interior 52, the electrons collide with metal atoms creating ions. The electrons are slowed down and the ions are accelerated away from the cathode 20 and the grid 21. Referring to FIG. 1B, a plot of potential distribution along the axis of the ion source furnace 18 is shown. Potential decreases from left to right along the abscissa and positions along the axis of the ion source furnace 18 are plotted as ordinates, the ordinate position being projected over to FIG. 1B from equipotential lines 22 of FIG. 1A. As can be seen, the potential initially rises rapidly from cathode 22 to a maximum at the grid 21. Thereafter, from the grid to the focal point 0, the potential decreases again to 0 so that if plotted graphically, within the potential on the abscissa and the distance from the cathode on the ordinate, the resulting curve resembles an hyperbola. The result is a spherically converging electric field.

Most substances have maximum ionization cross-section for electrons of about 50 to 100 ev. energy. I therefore use about a 40-50 volt drop between the cathode and the plate, and I keep the drop in electron energy inside the cylinder area 52 in which the spherical field is maintained by the unevenly wound helix 58 in the range of about 15-20 volts. The rest of the voltage drop is between the lower end of cylinder 64 and the nozzle 24, this nozzle being kept at cathode potential. As a consequence, most of the ionization occurring in the furnace cavity 52 takes place inside of the cylinder 64, wherein the cone of the converging ions has a large volume. Relatively very little ionization occurs between the lower end of cylinder 64 and the nozzle 24, where the electrons are particularly slow moving and which anyway has a very small volume. Therefore, the ion current produced by the region 18 is very uniform in energy and the energy spread is only of the order of about 1520 ev. A voltage of this order can be applied to, for example, a tungsten helix of say about 0.012 inches diameter without overheating or other deleterious consequences.

As explained, the inner heater is a tungsten helix with unevenly spaced turns preferably of a shape such as that shown in FIG. 6. The refractory ceramic tube 64 into Which'helix 58 fits is preferably made of alumina or the like. Preferably this tube has an inner diameter of about mm. and is about 50 mm. in length, although those skilled in the art will appreciate that the final dimensions of the ceramic tube are, of course, determined by the size of the helix desired.

The arrangement of the furnace is such that very high temperatures can be maintained within the region 52. The cavity 52 is heated not only by the helix 58, which serves as an inner heater for the furnace, but also by an outer heater which is more particularly shown in FIGS. 7A and 7B.

The outer heater is of the squirrel cage type. Tungsten rods 75 are hydrogen or argon arc welded into molybdenum rings 74 and 66. As this heater will expand appreciably, it must be held elastically. I therefore use a bellows 35 made up by welding together molybdenum rings inside and outside. The top of the bellows is in turn welded to the top plate 79. Circumferentially surrounding this outer heater is a pyrophyllite or lava ceramic split cylinder 34. As further insulation, a molybdenum sheet 33 serves as a heat radiation screen. This sheet 33 is simply inserted along the inside wall of the ceramic insulating cylinder 34 and can be corrugated so as to minimize contact with inner walls of split cylinder 34. The outer heater is energized from leads 67 and 71, lead 67 serving as the positive end and 71 as the negative end. These leads are fastened into copper sleeves. Sleeve is brazed at the point 69 to bottom molybdenum terminal ring of outer heater 74. Into copper sleeve 70 is fitted cable 71. Similarly, there is brazed to the top molybdenum terminal ring 79 of the outer heater a copper sleeve 72 at point 68; into this sleeve 72 is fitted cable 67. Ion exit port 60 in the bottom molybdenum terminal ring 74 has outwardly sloping sides preferably arranged at an angle of about 45 with the vertical.

At the top of cylinder 64 (FIG. 2) is mounted molybdenum ring 65, across which are welded fine tungsten wires 63 in a spaced, parallel configuration, as shown in FIGS. 5 and 6. These tungsten wires are arranged so as to be slightly dished and have a radius of curvature, say, of about 1 inch. This structure serves as the accelerating grid 21 and is energized by lead 40 which is the positive lead to helix 58. Lead 39, at the lower end of helix 58, passes by the accelerating grid structure 21 without contacting same. For this purpose, a small concave dish 31 is cut in the outside edge of ring 65 at the point where lead 39 passes out by the accelerating grid 21. In the same manner the point opposite where the dish 31 is placed is also slightly dished on the inside edge of ring 65. This dish 40 serves as the point where contact is made between lead 40 and the tungsten wires 63 of the accelerating grid 21.

Spaced above accelerating grid 21 at an equal distance therefrom is the cathode 20 which consists of a so-called Coolidge spiral as shown in FIG. 4. Behind the cathode is a molybdenum disc 47 which serves as a cathode holder. Cathode heater leads 54 are Welded to the disc 47 and serve to energize the cathode. Center hole-49 in cathode holder 47 serves as .an opening to receive lead 37 which directly energizes cathode 20. The end 51 of Coolidge spiral 20 is welded to the edge of the disc 47 at point 15.

The cathode assembly, comprising leads 54, disc 47, spiral 20 and lead 37, is mounted in an insulating plug 41 by means of leads 54 (see FIG. 3). Plug 41 is a circular disc shaped ceramic body thickened in its midportion; through the mid-portion leads are fixed. Also through the plug 41 mid-portion are fixed leads 39 and 40 for the inner heater helix, which completes the assembly of the furnace and inner heater assembly.

Note in FIG. 3 that the insulating plug 41 is so shaped as to fit inside the mouth 73 of the outer heater assembly (shown in FIG. 7A).

The relationship between the inner heater, outer heater and the rest of the ion source and first lens is shown in FIGS. 3 and 8 and FIG. 8A. The inner heater and outer heater assemblies are clamped together by means of the four suspension clamp rods 62. These rods are fitted through extractor plate 42 of the ion source. A washer or support platform 88 fits inside the base of the extractor plate 42 and serves .as a stage upon which rests the assembled outer heater (shown in FIG. 7). Mounted in the mid-portion of extractor plate 42 over ion exit port 61 is the nozzle 24. The lower outside edges of exit port 61 are rounded and polished. Observe that theinner and outer heater are concentrically positioned with respect to each other and have a common axis, this axis being positioned directly over the nozzle 24.

In FIG. 3 is shown one way of mounting the outer and inner heater combined assembly. Top plate 43 is laid across the insulating plug 41 and the combined assembly of the outer and inner heater is secured by the clamping action of the rods 62 with their bolts 85.

In FIG. 8 and FIG. 8A is shown a second way of mounting the combined outer and inner heater assembly.

Here, instead of plug 41 and plate 43, there is employed a threaded insulating plug 90 and a main assembly ring' 93 having a tapped central bore with threads matching those of plug 90; By this arrangement, the inner heater Circumferentially positioned, so as to be immediately outside of the combined assembly of the outer and inner heater, is an electromagnet coil 32 whichserves .to force the electrons to pendulate along the magnetic lines of force as indicated in FIG..2 by arrows within theinner heater space 52. This coil 32 and the inner and outer heaters are mounted in a frame assembly means'. The whole frame is fixed to the grounded lens electrode plate 48 of objective lens assembly 57.' Positioned in the edge of this electrode plate 48 at 90. intervalsare the four main assembly rods 80. Below the level of the electrode plate 48, these rods. have positioned about them a non-magnetic spacing sleeve 89 as a means for positioning the rods.

Above the plate each rod is fitted with a sleeve 83 of an insulating ceramic material. Near the top of each such sleeve 83 a channel iscut over which a main assembly ring 86 is slipped so as to form a platform, whereon the com bined outer and inner heater assembly rests. To rigidify and fix the main assembly ring 86, secondary sleeves 84 are each positioned over sleeves 83.- Then washers 81 are placed over the top of each main assembly rod80 and the structure is secured by me-ans'of bolts 82 screwed down over washers 81.

The inner heater cathode, the grid,-the helix and the cylinder 64 can be easily inserted into or removed from the frame assembly, depending upon whether the arrangement in FIG. 3 or that in FIG. v8 and FIG. 8A is used.

Grid 21 should have veryfine wires in its mesh with large clearances so that at least about 90 to 95 percent of l the cross-sectional area of the grid is open space. The grid should be placed as close to the cathode as possible, say not farther apart than about 0.020 inch. This has the advantage of large saturation currents. It also enhances cathode longevity, for very few ions are generated in the small gap between the cathode and the grid. Outside the grid the electrons are slowed down, and the ions are accelerated away from the cathode and the grid. Thus, there is very little wear by sputtering of the cathode and virtually none whatever of the grid.

At the very small vapor pressures which can be maintained with such materials as boron or carbon even in a very hot furnace, the mean free path for ionizing collisions is rather long. As an example, the ionization crosssections are usually the order of a few 10 cm. Ata pressure of one micron at 2700 .K this gives ionizing paths of about 1000. cm.; i.e., one electronin 400 would produce an ion in a length of about 2.5 cm. In order to increase the ionization efiiciency, I let the electrons pendulate between the cathode and the nozzle .at the cathode potential in a strong magnetic field. This is another reason for making the grid with a clearance of 90-95 percent so that the electrons can perform many pendulations before being absorbed by the grid. This system is hence very efiicient from an energy per ion point of. view.

When the ion source is operated with a gas, .suchas hydrogen or nickelcarbonyl, the gas can be fed through a gas inlet port which can be a metal capill-arytube 36 (see FIG. 3), from whence it enters the heating chamber. 52 through plug 41. When the ion gun is operated with such a gas, the. outer heater assembly. isusu ally notneces sary since lower temperaturesare sufiicient. The heliX 58 is still necessary since it has thefunction not only of; heating but also of mvaintaining'the spherically converging '70 potential distribution in the chamber 52.

Ion source materials "which do not melt at operating temperatures, even when the'o'uter heater is used, such as carbon in the form of Aquadag, can be applied as a paste to the inside. of the ceramic tube 64 before the operation of the furnace is initiated. Thus materials such as boron or carbon are placed in direct contact with the hot tungsten helix 58. Similarly, beryllium and silver, which are molten at the operating temperatures achieved by the furnace, can also be applied to the inside walls. Another technique is to provide a well 91 (see FIG. 8) on the basebf cylinder 64 wherein a supply of molten materials such as molten metal can be kept during operation of the furnace. Still another means is to wind a thin wire or strip of metal around the tungsten spirally wound heater 58; thus, aluminum and silver can be handled as they have the right amount of limited solubility in tungsten for wetting the hot wire without forming large drops. A metal such as beryllium behaves similarly to aluminum to this respect. When non-gaseous materials are to be ionized in cavity 52, the insulating plug can be redesigned so as to screw into' the assembly ring. Thus, in FIG. 8 is shown an assembly whereby a screw-in plug 90.is secured into a main assembly ring 93, thereby enabling one to periodically introduce a fresh supplyof vmaterial to be ionized.

After leaving nozzle 24, the ion current generated by the ion furnace enters the electron optical system. This system comprises, in general, three electrostatic electron lenses; Magnetic ion lenses are useless for ions, as even for protons their strength is less by a factor of 43 compared .withtheir strength for electrons. The electrostatic electron lens system of this invention is better understood ally parallel ion beam, which is scanned by deflection system 11, across the area of a projector lens 13 to produce a small image (not shown). This image is perhaps about 0.5 .square centimeter in area in the embodiment depicted here.

FIGURE 10shows the image etched mask (not shown) being produced on a reduced scale. This operation is somewhat more complicated than that illustrated in FIG. 9. In .order to concentrate the'ion beam from point 0 into the final small final picture pattern, it is necessary to introduce a large field lens 12, near the position of the transparencyor etched mask 19. Such field lens 12 has a usefulfield as large as the image on a transparency 19.

In this operation, the objective lens 10 is inactive so that the whole transparency 19 is flooded with the ionbeam from O. i

In bo th .the scanning operation (FIG. 9) and in the operation of reducing a transparency (FIG.,10), a projectionlens 13 .is used to focus a beam upon a plate or surface ,14. Observethat while the field lens 12 is active when a transparency is being reduced, this same fieldlens 12 is inactive during a scanning operation. Similarly, in reducing .a transparency, the objective lens 10 is inactive, though in a scanningoperationpin is active. Note thatthe scanning operation requires the use of conventional.deflectorsv 11 :whichcan be either electrostatic or electromagnetic. in operation, as those skilled in the art will appreciate.

In actual practice theseparate optical systemsdepicted in FIGS. 9 and 10 arecombined in a common system comprising three electrostatic electron lensesas indicated (pyrophyllite or lava) upper cylinder 44 and a lower cylinder 45 of the same material. Between these two cylinders is placed the actual lens electrode 46 for the objective lens.

The lower half of the electron optical system is separately mounted. It is contained in a tube 100 such as brass, say, approximately 4 inches in diameter (see FIG. 11). The separate tube 100 is used because of the large size of the field lens assembly 94. The field lens electrode 95 is mounted in a ceramic cylindrical electrode insulator holder 96. At each end of the holder are placed, respectively, third grounded lens electrode 97 and fourth grounded lens electrode 98. A circular orifice 101 is provided in the Wall of tube 100 for lead 99 which energizes lens electrode 95. v

The projection lens assembly 103 is also mounted in the brass tube 100. A non-magnetic cylindrical electrode support 106 ring holds a brass or bronze disc 105 which slips inside of tube 100. The end of tube 100 is fitted with a grounded lens electrode 104. Holding the lens electrode 108 itself are two non-magnetic insulating electrode holder rings 107 and 109. Lead 110 energizes lens electrode 108.

. Not shown in the drawings is the deflector system which is conventional in design and is placed quite close to the exit of the objective lens. Also not shown in the drawings is the means for fixing and centering the upper half of the system containing the ion source and the objective lens, as well as the means for fixing and centering the lower half of the system containing the field lens and the projection lens. Also not shown in the drawings is the vacuum chamber and pumping system. The entire apparatus is contained in a vacuum chamber when mounted as for use, the vacuum being at least of the order of mm. Hg.

While my invention can handle all sorts of materials and ions, I wish to point out that in any given situation where one decides to use a single type of ion or ions, some simplification of the apparatus of this invention is possible, as those skilled in the art will readily appreciate. I prefer to use ions and ion sources which are-easily handled and give the best nuclei for ionic writing or machining. In general, metals which have gaseous compounds seem to be most easy to handle. Thus, while silver produces a good ion source, compounds such as nickel-carbonyl, tungsten hexachloride, or the like, are easier to handle. However, such compounds containing the carbonyl or halide anion tend to corrodetungsten cathodes and electrodes and so, for some purposes, these gaseous compounds are not desirable.

Generally, satisfactory operation is achieved by using not more than 1015 kilovolts. Above 30 kilovolts, electrostatic lenses are liable to suifer from frequent breakdowns. I prefer to use all lens potentialssupplied from a single potentiometer. This insures not only insensitivity to voltage fluctuations, but also that all ions, whatever their mass, will be focussed alike. Hence, there is no need to observe high purity.

The circuitry is, in general, conventional in construc-' tion. Perhaps the most difiicult part of the electric circuitry is that providing the heating for the cathode of the ion source, and for the outer and inner heater,

respectively. The inner heater should require not more than about 5 amperes at 12 to 20 volts. The Wire thickness must of course be adjusted to avoid any overloading. The outer heater generally should be capable Otf carrying approximately 1100 arnperes at about 1.volt drop. While' this outer heater can use alternating current, the inner heater must be operated with direct current because this inner heater also supplies the voltage drop for shaping the field. Note that helix 58 provides a field which corresponds to the l/ r law.

I claim:

1. In an apparatus for producing ion beams employing spherically converging electrostatic field between said accelerating grid and said nozzle; and means for maintaining a substantially axial magnetic field in the region between said accelerating grid and said nozzle so as to produce pendulation of electrons in said region.

- 2. In an apparatus for producing ion beams, the combination comprising a means of emitting electrons, an accelerating grid, and a nozzle, said means of emitting electrons, said accelerating grid, and said nozzle all having a common axis; a source of ionizable material in the region of said accelerating grid and said nozzle; means for maintaining approximately the same potential on both nozzle and cathode; means for maintaining an approximately spherically converging electrostatic field between said accelerating grid and said nozzle including a current carrying coil of wire wound in a helix about the said common axis with continually diminishing spacing between turns from the end nearest the grid to the end nearest the nozzle; and means for maintaining a substantially axial magnetic field in the region between said accelerating grid and said nozzle so as to produce pendulation of electrons in said region.

3. In an apparatus for producing ion beams, the combination comprising a means for emitting electrons, an accelerating grid, and a nozzle, said means for emitting electrons, said accelerating grid, and said nozzle all having a common axis; a source of ionizable material in the region of said accelerating grid and said nozzle; means for maintaining such ionizable material in a vaporized condition in the region of said accelerating grid and said nozzle; means for maintaining approximately-the same potential on both nozzle and cathode; means for maintaining ,an approximately spherically converging electrostatic field between said accelerating grid -anrl said nozzle by using unevenly spaced turns of wire wound about the said common axis; and means for maintaining a substantially axial magnetic field in the region between said accelerating grid and said nozzle so as to produce pendulation of electrons in said region.

4. In an apparatus for producing ion beams, the combination comprising a means of emitting electrons, an accelerating grid, and a nozzle, said means of emitting electrons, said accelerating grid, [and said nozzle all having'a common axis; a source of ionizable gas in the region of said accelerating grid and said nozzle; means for maintaining approximately the same potential on both nozzle and cathode; means for maintaining an approximately spherically converging electrostatic field between said accelerating grid and said nozzle by using a current carrying coilof wire wound in a helix about the said common axis with continually diminishing spacing between turns firom the end nearest the grid to the end nearestthe nozzle; and means for maintaining a substanti-ally axial magnetic field in the region between said accelerating grid and said nozzle so as to produce pendulation of electrons in such region.

5. An apparatus for producing ion beams comprising in combination, a furnace for vaporizing solid materials having an inner heater containing a spirally wound elec- Il'iC'qll heating element and an outer heater containing a --tween turns about said common longitudinal central axis so as to produce a spherically converging electrostatic field; an electromagnet coil circumferentially mounted about said outer heater; a low energy electron source positioned in the upper (positive) end of said inner heater comprising a cathode heater, a cathode and an accelerating 9 10 grid; and an electrostatic lens system for systematically 2,719,925 10/ 1955 Oppenheimer 250-495 focusing, accelerating and deflecting the ions issuing from 2,809,314 10/ 1957 Herb 25041.9 the lower (negative) end of said inner heater, 2,817,033 12/1957 Brewer 313-83 X 2,826,708 3/1958 Foster 25041.9 References Cited by the Examiner 5 UNITED STATES PATENTS JAMES W. LAWRENCE, Przmary Exammer. 2 392 243 1 194 Hillier 250 49 5 RALPH G. NILSON, GEORGE WESTBY, Examiners. 2,570,124 10/1951 Hernqvist 25041.9 W, F, LINDQUIST, S. SCHLOSSER,

2,714,679 8/1955 Van de Graaif et a1. 25041.9 10 Assistant Examiners. 

2. IN AN APPARATUS FOR PRODUCING ION BEAMS, THE COMBINATION COMPRISING A MEANS OF EMITTING ELECTRONS, AN ACCELERATING GRID, AND A NOZZLE, SAID MEANS OF EMITTING ELECTRONS, SAID ACCELERATING GRID, AND SAID NOZZLE ALL HAVING A COMMON AXIS; A SOURCE OF IONIZABLE MATERIAL IN THE REGION OF SAID ACCELERATING GRID AND SAID NOZZLE; MEANS FOR MAINTAINING APPROXIMATELY THE SAME POTENTIAL ON BOTH NOZZLE AND CATHODE; MEANS FOR MAINTAINING AN APPROXIMATELY SPHERICALLY CONVERGING ELECTROSTATIC FIELD BETWEEN SAID ACCELERATING GRID AND SAID NOZZLE INCLUDING A CURRENT CARRYING COIL OF WIRE WOUND IN A HELIX ABOUT THE SAID COMMON AXIS WITH CONTINUALLY DIMINISHING SPACING BETWEEN TURNS FROM THE END NEAREST THE GRID TO THE END NEAREST THE NOZZLE; AND MEANS FOR MAINTAINING A SUBSTANTIALLY AXIAL MAGNETIC FIELD IN THE REGION BETWEEN SAID ACCELERATING GRID AND SAID NOZZLE SO AS TO PRODUCE PENDULATION OF ELECTRONS IN SAID REGION. 